The present invention relates to improvements in known differential units of a type which provide for a locking out of differential action under certain adverse driving conditions. More specifically, the invention provides for an improved assembly which includes two stages of operaton: a first stage in which there is no tendency to lock out normal differential action, and a second stage in which there is a tendency to lock out differential action of the differential unit.
It is known in this art to provide for various locking means in differential units for limiting or preventing normal differential action of such units. A typical arrangement for limiting differential action is illustrated and described in U.S. Pat. No. 2,855,805 wherein a known design of differential unit is provided with a face coupling assembly for transmitting torque between one of the side gears of the differential and its associated axle shaft. The function of the face coupling assembly in this known arrangement is to develop an axial thrust between mating members of the face coupling to thereby move working components of the differential unit to positions which tend to resist normal differential action. Axial thrust is developed from positive pressure angles included on mating surfaces of the face coupling members (see FIG. 2 of the patent), and the basic arrangement is one which provides for varying degrees of axial thrust within the differential unit in accordance with imbalances in driving torque delivered to the separate axle shafts associated with the unit.
Known differential units of the type just described have a characteristic of tending to resist differential action under many driving conditions in which full differential action is needed. Thus, there may be a tendency for a limited slip device or arrangement to resist full differential action when a vehicle is traveling through a curved path so as to cause an outside wheel of an axle system to rotate at a faster rate than an inside wheel of the same system. The differential rate of rotation is sufficient, in many limited slip differential units, to initiate a slight resistance to full differential action, and this can result in unwanted wear and noise in the differential units.
In contrast to prior art arrangements which provide for automatic control of differential action, the present invention provides for an automatic control which does not tend to limit or resist full differential action during a wide range of normal driving conditions for the axle system with which it is associated. Thus, there is no tendency, with the differential unit of the present invention, for a limited slip device to resist normal overriding of one wheel relative to another in the axle system, and full differential action is obtained at all times in a first stage of operation of the differential unit. On the other hand, extreme imbalances in torque transmission will result in an automatic shifting of certain components of the differential unit to a second stage of operation which provides for a resistance to differential action when it is most needed.
In accordance with the present invention, a novel face coupling assembly is included in a known type of differential unit, and the novel face coupling assembly includes two mating sections which can be axially shifted to separate positions of driving contact so as to provide for two distinct driving characteristics through the differential unit. In addition, there is provided a hydraulically actuated control means for moving the mating sections of the face coupling assembly between their separate positions for transmitting torque.
In a preferred embodiment of the invention, a known design of differential unit is provided with a face coupling assembly located between one of its side gears and an associated axle shaft for effecting a change in relative rotation between both axle shafts of the system in accordance with axial positioning of mating sections of the improved face coupling assembly. The face coupling assembly is of the type which includes a first coupling section having tooth formations formed on a face thereof for meshing with tooth formations formed on a corresponding face of a mating section. Each tooth formation, of both coupling sections, is provided with a zero pressure angle portion at a base thereof together with a positive pressure angle portion extending from the zero pressure angle portion to a tip thereof. The tooth formations of both of the coupling sections are shaped and dimensioned to mesh with each other in a driving relationship which includes (a) a first position in which zero pressure angle portions of mating tooth formations contact each other and (b) a second position in which positive pressure angle portions of mating tooth formations contact each other. In addition, a control means is included for effecting relative axial shifting of the first and second coupling sections of the assembly, and the control means can include a piston member fitted within the assembly and operatively connected to one of its coupling sections so as to urge that coupling section axially away from the other coupling section when a hydraulic force is applied to a working face of the piston member. Hydraulic force for actuating the control means can be developed by pumping action of gears contained within the differential unit.
These and other features and advantages of the present invention will become apparent in the more detailed description which follows and in that description reference will be made to the accompanying drawings as briefly described below.